A deadly strain of bird flu has claimed the lives of more than 50 people in Asian countries since early last year. The majority of recent cases have been in Vietnam -- 49 since December 2004.

Seventeen were fatal, and some have occurred in clusters, raising fear of a change in the transmission pattern.

Millions of chickens, ducks and geese have been killed in Vietnam, Cambodia and Thailand in an effort to stem the virus's spread.

The World Health Organization is concerned the virus might mutate into a form that can be passed from one person to another and create a global pandemic.

WHO spokesman Peter Cordingley says so far there is no conclusive proof of human-to-human transmission. "We have found a couple of cases that were very suspicious, but we couldn't actually hammer that nail home."

But human-to-human transmission may not be too far off. On May 14, Indonesia identified the bird flu virus in pigs on the densely populated island of Java. Pigs are genetically similar to humans and can carry both human and animal viruses. The fear is the bird flu virus in an infected pig could mutate and create a new strain that could spread from human to human.